Method and device for recording fingerprint data

ABSTRACT

In a method for recording reference fingerprint data for a person, at least two fingerprint images are recorded that depict at least partially different areas of one and the same finger of the person. Data from these images is stored in at least two alternative sets of reference fingerprint data that are to be used in checking the identity of the person. This method makes it possible to use a small sensor that only records partial fingerprints, both when recording reference data and when verifying, without the person having to make repeated attempts during the verification in order for the sensor to record the same area on the finger both when recording reference data and during the verification. In addition, a method is described for checking identity, in which method the fact is utilized that there are alternative sets of reference fingerprint data, and devices for carrying out the methods.

FIELD OF THE INVENTION

The present invention relates to a method and a device for recordingreference fingerprint data for a person. It further relates to a methodand a device for checking identity.

BACKGROUND OF THE INVENTION

It has been known for many years that fingerprints can be used to checka person's identity. For the checking, a current fingerprint is recordedfrom the person whose identity is to be checked and this is comparedwith previously recorded reference fingerprint data. The checking canconcern a verification of the identity of the person. In such a case,the current fingerprint is compared only with reference data for theperson that the person whose identity is to be verified is purporting tobe. The checking can also concern an identification of the person'sidentity. In such a case, the current fingerprint is compared withreference data for several different persons in order to determinewhether the current fingerprint comes from any of these persons.

There are different types of sensors for recording fingerprints. Thesensors can for example be optical, thermal, pressure-sensitive orcapacitive. Traditionally, the sensors have had a surface with a sizethat makes possible the recording of a complete fingerprint.Particularly with regard to capacitive sensors, that are manufacturedfrom silicon, the cost of a sensor is, however, greatly dependent uponthe size of the surface of the sensor. There is therefore a desire to beable to make the sensors smaller.

EP 0 813 164 describes a thermal sensor that only allows a part of afingerprint to be recorded. In order to record a complete fingerprint,the user passes his finger across the sensor which records images at ahigh frequency. The images, that partially overlap each other, are thencombined into an image of a complete fingerprint. The identity check iscarried out in the usual way on the basis of the complete fingerprint.

This sensor has the advantage that it has a smaller sensor surface thansensors that record a complete fingerprint with a single image. Thedisadvantage, however, is that it is more difficult to use, as the usermust pass his finger over the sensor at a relatively even speed in orderfor the image to be clear. If the user passes his finger over the sensorslowly or even stops, the result will be impaired as the sensor is basedon temperature differences that are evened out immediately if the fingeris held still.

WO 00/49944, that was filed by the Applicant of the present application,describes the use of a small sensor that records a partial fingerprint,that is only a part of a complete fingerprint. According to thisapplication, a reference fingerprint image is recorded using atraditional “large” sensor or using a small sensor with the imagescombined into an image of a complete fingerprint. For the identitycheck, on the other hand, only an image of a partial fingerprint isrecorded. This partial image is compared with the considerably largerreference image in all possible relative positions in order to checkwhether the partial image corresponds to any partial area of thereference image and can thereby be assumed to come from the same personas the reference image.

This method makes it possible for the user to hold his finger still onthe sensor during the identity check. The comparison of all overlappingpositions requires, however, a certain processor capacity and can take arelatively long time. In addition, an essentially complete referencefingerprint must be stored. This can be a disadvantage, for example whenthe check is to be carried out on a smart card which normally haslimited processor and memory capacity.

In other known methods for checking identity using fingerprints, acomplete fingerprint is, however, not normally stored as referencefingerprint data, but instead the most relevant information is extractedfrom the fingerprint and saved as reference fingerprint data. Forexample, different partial areas of an image of a fingerprint can besaved as reference data.

WO 01/84494, that was also filed by the Applicant of the presentapplication, describes in addition how the identity check can be dividedbetween a smart card and a processing unit in order to minimize theoperations that need to be carried out on the smart card and thattherefore require processor capacity. More specifically, the referencedata is divided in this instance into a public part, that can leave thesmart card, and a private part, that cannot leave the smart card. Thepublic part contains a partial area and coordinates that indicate thelocation of additional partial areas that are to be found in the privatepart. To carry out an identity check, a current fingerprint is recordedfor the person whose identity is to be checked. The public partial areais transmitted from the smart card to the processing unit in which it iscompared with the current fingerprint in order to check whether itmatches anywhere in this. If such is the case, the relative positions(translation and rotation) of the reference data and the currentfingerprint are determined. The coordinates in the public part of thereference data are hereafter used to determine the partial area of thecurrent fingerprint that is to be matched against the partial areas inthe private part of the reference data. These partial areas of thecurrent fingerprint are sent to the smart card where the identity checkis concluded. As an alternative to the public partial area, a set ofspecial characteristics, so-called features, can be stored in the publicpart of the reference data and used in order to determine the relativepositions of the reference data and the current finger-print.

If a small sensor, that is a sensor that only records a partialfingerprint, is used for this application, the problem arises that itcan happen that the user places his finger on the sensor in such aposition that only some of the partial areas or none of these at all liewithin the sensor surface area. If this occurs, the identity check willnot succeed due to the fact that there is an insufficient number ofpartial areas to compare. The user must then carry out a new attemptwith his finger in a new position. A number of attempts can be requiredand this is of course inconvenient for the user.

SE 0102376-1, which was filed on 29 June 2001 by the same Applicant asin the present application and which was thus not publicly available atthe time of filing the present application, proposes a solution to thisproblem which consists of checking how well the reference data isaligned with the current fingerprint. More specifically, on the basis ofwhere the public partial area is located in the current fingerprint andusing the coordinates for the private partial areas, it is possible towork out whether the private partial areas are located within the sensorsurface area and thus whether the identity check has the possibility ofsuccess. If such is not the case, an indication can be given to the userregarding how he or she is to move his/her finger in relation to thesensor in order to improve the alignment.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the problem of the userpossibly needing several attempts to identify himself with his finger indifferent positions on the sensor where this is of the type that onlyrecords a partial fingerprint.

The object is achieved wholly or partially by means of a method and adevice for recording reference fingerprint data according to claims 1,11 and 14 respectively, and a method and a device for checking identityaccording to claims 16 and 24 respectively.

The invention thus relates to a method for recording referencefingerprint data for a person, comprising the steps of receiving atleast two fingerprint images that depict at least partially differentareas of one and the same finger of the person, and storing data fromsaid at least two fingerprint images as at least two alternative sets ofreference fingerprint data, that comprise data from at least partiallydifferent areas of the finger, for use in checking the identity of theperson, wherein at least two alternative sets of reference fingerprintdata are stored in a first unit in which checking of the person'sidentity is to be carried out, each of the sets of reference fingerprintdata comprising public data that can be communicated from the first unitto a second unit during the check, and also private data that is usedexclusively in the first unit during the check.

As more than one set of reference fingerprint data is stored, a largerpart of the person's fingerprint is imaged in total, and the chances arethereby increased of a partial fingerprint that is taken during anidentity check lying within the imaged part of the fingerprint, which inturn leads to fewer attempts, and optimally only one attempt, needing tobe made in order, for example, to verify a person's identity.

Instead of the user needing to move his finger around and try severaldifferent positions on the sensor on each occasion when he wants toverify his identity, this need now only be carried out on one occasionand that is when recording the reference data. This should be aconsiderable improvement for the user.

In addition, as alternative sets of reference fingerprint data arestored, the same algorithms can be used both for a small sensor thatrecords a partial fingerprint and for a large sensor that records acomplete fingerprint and only needs one set of reference fingerprintdata as the problem of the position of the finger on the sensor isreduced.

The alternative sets of reference fingerprint data can be stored invarious ways. They can, for example, be stored as completely separatesets that do not share any data at all. If overlapping data is includedin the alternative sets, this data is thus stored once for each set. Thedifferent sets can alternatively constitute different subsets of acommonly stored quantity of data. If overlapping data is included in thealternative sets, in this case it thus only needs to be stored once.

The different sets of reference fingerprint data can be used forchecking identity. According to the invention, a method for checkingidentity comprises more specifically the steps of receiving a currentfingerprint image that depicts a part of a finger of a person whoseidentity is to be checked; comparing data from the fingerprint imagewith data from at least a first set of reference fingerprint data of atleast two previously stored sets of reference fingerprint data thatrelates to at least partially different areas of one and the same fingerof a person whose identity is known; and, on the basis of the result ofthe comparison, determining which of said at least two alternative setsof reference fingerprint data is to be used for checking the identity.In checking the identity it is thus determined whether the person whoseidentity is to be checked has the same fingerprint as the person whoseidentity is known, that is whether they are one and the same person.

Each of the sets of reference fingerprint data can comprise a pluralityof partial areas of the corresponding fingerprint image. This makespossible storage and matching of the reference fingerprint data on, forexample, a smart card and other units with limited processor and memorycapacity.

In addition, one of the partial areas can be used to determine which setof reference fingerprint data is to be used for an identity check. Thusno exhaustive checking against all data in all the sets of referencedata needs to be carried out and the time required for the identitycheck is thus increased very little, in spite of the fact that there areseveral different sets of reference fingerprint data. This can be aparticular advantage when the identity check is carried out insituations where there is already a risk of it being slow, such asidentity checks on smart cards, via networks or in databases where thecheck must be carried out against reference data from several persons.

As an alternative to using a partial area in order to determine whichset of reference fingerprint data is to be used for an identity check,features can be used. As mentioned above, a feature is a specialcharacteristic in the fingerprint, for example a point where a ridgeends or a point where a ridge divides. Such characteristics are alsoknown as “minutia points”. In this case, the positions and/or otherinformation are extracted, such as the angle of/or type of feature fromthe fingerprint. This information is then stored in the public part ofthe reference fingerprint data instead of the public partial area.

The methods according to the invention can be implemented in devicesthat comprise a signal processor. The signal processor can be aprocessor that is provided with suitable software for carrying out themethod. It can also comprise application-specific hardware, such as anASIC, a programmable logical circuit, such as an FPGA, or digital oranalog circuits or some suitable combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail by means ofan embodiment and with reference to the accompanying drawings, in whichFIG. 1 a shows schematically a fingerprint in which are marked differentareas that are imaged during the recording of alternative sets ofreference fingerprint data.

FIG. 1 b shows schematically the same fingerprint as in FIG. 1 a butwith only one imaged area marked.

FIG. 1 c shows schematically the same fingerprint as in FIG. 1 a, but inwhich is marked an area that is imaged during the identity check.

FIG. 2 shows schematically a system in which the methods and the devicesaccording to the invention can be used.

FIG. 3 is a flow chart and shows an example of a method for recordingreference fingerprint data.

FIG. 4 is a flow chart and shows an example of a method for checkingidentity.

DETAILED DESCRIPTION OF AN EMBODIMENT

The present invention is based on the idea of recording from one and thesame finger at least two different sets of reference fingerprint data,which correspond at least partially to different areas of the finger.This idea is illustrated in FIG. 1 a, which shows schematically acomplete fingerprint 1, which is recorded from a person's fingertip andin which are marked three areas 2 a-2 c that are imaged using a “small”sensor when recording reference fingerprint data. The three areascorrespond to three partial fingerprints. As shown in FIG. 1 a, theareas 2 a-2 c can overlap each other. However, they do not need tooverlap.

As has been described by way of introduction, the whole of the areas 2a-2 c do not normally need to be stored as reference fingerprint data,but only extracted data. FIG. 1 b shows the same fingerprint as in FIG.1 a, but only the upper area 2 a is marked. In this area six partialareas A-F are indicated. These partial areas can be stored as referencefingerprint data.

When the identity check is to be carried out, it is usually sufficientfor the user to place his finger once on the small sensor. FIG. 1 cshows again the same fingerprint 1 as in FIG. 1 a. A current area 3 thatis imaged by the small sensor is marked in the fingerprint. This currentarea corresponds best to the upper area 2 a in FIG. 1 a, which forexample can be determined by a partial area of each of the areas 2 a-2 cin FIG. 1 a being compared with the current area. This means in turnthat it is a set of reference fingerprint data that essentiallycorresponds to the uppermost reference area that is to be used for theactual identity check.

If only the middle or the lower area had been imaged during therecording of reference fingerprint data, the identity check would nothave the possibility of success as the current area would not then haveoverlapped the area that was imaged during the recording of thereference fingerprint data and it would therefore not be possible todetermine that it came from the same person. The user would have had totry at least once more with his finger in a different position on thesensor.

FIG. 2 shows a system that can, but need not, be used for both recordingof reference fingerprint data and checking identity. The systemcomprises a fingerprint sensor 21, a computer unit 22 that constitutesan example of a signal processor, a smart card reader 23 and a smartcard 24.

The sensor 21 can be optical, capacitive, thermal, pressure sensitive orof any other suitable type. Its sensor surface is such a size that itcan only record a partial fingerprint from a normal person's fingertip.A sensor for recording a complete fingerprint is a size of at least200×200 pixels with a resolution of 500 dpi. Such a sensor records animage of the whole area of the fingertip that comes into contact with asurface when the fingertip is pressed against a surface that is the samesize or larger than the fingertip. A small sensor for a partialfingerprint can be a size of, for example, 128×128 pixels or less. Thissensor will only image a part of the area on the fingertip that comesinto contact with a surface when the fingertip is pressed against this.The sensor 21 can be separate or integrated with the computer unit 22 orthe smart card reader 23 or the smart card 24.

The computer unit 22 can be an ordinary general-purpose computer, forexample a PC. Alternatively, it can be a computer unit 22 that isdedicated to processing fingerprints. It can, for example, comprise aprocessor with program memory and working memory or can comprisespecially adapted hardware, such as an ASIC (Application SpecificIntegrated Circuit) or a programmable logical circuit, such as an FPGA(Field Programmable Gate Array) or digital or analog circuits or somesuitable combination thereof.

The smart card reader 23 can be any commercially available smart cardreader. Alternatively, it can be a specially adapted smart card reader.It can be a separate unit or can be integrated with the computer unit22.

The smart card 24 can be any type of smart card on which referencefingerprint data can be stored. The smart card has a signal-processingunit that comprises a processor 25, a memory 26 for storing referencefingerprint data which is extracted from a reference fingerprint fromthe holder of the smart card, and a working memory 27, in addition tocommunication circuits 28 that make possible communication between thesmart card reader 23 and the smart card 24. The communication circuits28 may, but need not, require contact between the smart card and thereader.

The system can further comprise indicating means 29 which are intendedto provide information to the user. The indicating means 29 can, forexample, consist of a display on which messages or signals aredisplayed, light-emitting diodes or some form of sound-generatingcircuit. In FIG. 2 the indicating means are shown on the computer unit22, but they can also be arranged on the sensor 21.

In the following, it will be described how recording of referencefingerprint data can be carried out using the system in FIG. 2 withreference to the flow chart in FIG. 3. In this example, the data that isextracted from the images consists of partial areas, and of thereference fingerprint data is stored on a smart card.

The reference fingerprint data is described in the following as atemplate. A template can comprise a complete or a partial fingerprint inthe “raw” or unprocessed form in which it is recorded. Normally,however, a template contains selected, processed and compressed datafrom the recorded fingerprint. That is the case in this example.

The method starts with the user, that is the person for whom referencefingerprint data is to be recorded, placing his fingertip on the sensor21. An image in grayscale of an area, for example the area 2 a in FIG.1, of the fingerprint is recorded, step 30. The image constitutes adigital representation of the fingerprint area.

In the next step 31 it is checked whether the image can be accepted.This check can, for example, comprise one or more of the elements below.

The computer unit 22 can, for example, check the recorded image so thatit is ensured that there is actually a fingerprint in the image, thatthe fingerprint takes up a sufficiently large part of the image and thatthe fingerprint is sufficiently clear.

The computer unit 22 can, in addition, check whether the user hasapplied his finger with sufficient pressure on the sensor and that anymoisture on the user's finger has not made it impossible for the sensor21 to distinguish between ridges and grooves in the fingerprint.

In addition, the computer unit 22 can binarize the recorded image andcompare it with one or more possibly previously recorded images in orderto check whether the most recently recorded image differs sufficientlyfrom the previously recorded images. If, for example, three sets ofreference fingerprint data are to be recorded on the basis of threereference images, these should preferably differ so much that togetherthey cover a considerably larger area than a single image so that thepossibiities thereby increase that a later image taken in connectionwith an identity check, such as the image of the area 3 in FIG. 1 c,lies within the area that is covered by the reference images so that theidentity check has the possibility of success.

If the recorded image cannot be accepted, the computer unit 22 gives asignal to the user concerning this via the indicating means 29, step 32.In the simplest case, the signal can just be a signal to the effect thatthe image was not accepted. The signal can, however, also contain anindication of the reason why the image cannot be accepted and/or anindication of what the user should do in order to remedy the problem.For example, if the image that was not accepted overlaps previouslyrecorded images by too great an extent, the user can receive anindication of in which direction his finger should be moved. Thecomputer unit 22 can, for example, calculate this by comparing a partialarea of a previously recorded and accepted image with the most recentlyrecorded image and determining where in the most recently recorded imagethe partial area fits. In this way, the relative positions of the mostrecently recorded image and the previously recorded and accepted imageare determined, and the computer unit can thereby determine how thefinger should be moved in relation to the sensor 21 in order to reducethe overlap.

Subsequently the flow returns to step 30 for recording a new image.

If the recorded image can be accepted, in the next step 33 it isdetermined which partial areas are to be incorporated in the template.

First a partial area of the image, below called a public partial area,is selected from the image for storage in a public part of the template.The partial area can be selected in various ways. One way is to use thefollowing three quality criteria: 1) Distinctness, that is how easy apartial area is to binarize, 2) uniqueness, that is how unique a partialarea is, and 3) geographical location, that is where a partial area islocated in the fingerprint.

The uniqueness can, for example, be checked by correlating the partialarea with the surroundings and selecting a partial area with littlecorrelation with the surroundings. Alternatively, partial areas with oneor more features can be sought.

Regarding the geographical location, partial areas are preferred in thecenter of the image as this is where the fingerprint is least deformedwhen the user presses his finger against the sensor with various degreesof pressure and, in addition, there is a greater chance that the partialareas will be included in a later recorded image that is to be used forchecking identity.

Depending upon the relationship between the size of the sensor, thenumber of partial areas and the size of the partial areas, theabove-mentioned criteria can be more or less significant. It can happenthat the partial areas overlap each other and even cover essentially thewhole image. It can still be an advantage to select partial areas as thesame algorithms for recording reference fingerprint data and forchecking identity can then be used for images from sensors of differentsizes.

As an alternative to selecting partial areas by means of various qualitycriteria, fixed partial areas can be selected.

In any case, a partial area is selected to form the public partial area.Preferably only one public partial area is selected so that as littleinformation as possible about the user's fingerprint is available in thepublic part of the template. However, more public partial areas can beselected in order to achieve a more certain correlation between thepublic part of the template and a current fingerprint and thereby toachieve a more certain orientation of the template in relation to thecurrent fingerprint.

When the public partial area has been selected, at least one butpreferably a plurality of partial areas, below called private partialareas, is determined or extracted, for storage in a private part of thetemplate on the smart card 24. The private partial areas are preferablyselected according to the same quality criteria as the public partialarea(s). Four to six private partial areas are preferably selected. Moreor fewer partial areas can be selected depending upon the required levelof security, the required speed in matching on the smart card 24 and theavailable processor capacity of the smart card 24.

The size of the selected public and private partial areas is in thisexample (500 dpi), 48×48 pixels, but can easily be adapted by a personskilled in the art to suit the current requirements.

In association with the private partial areas being selected, theirlocation in relation to a reference point is also determined. Thereference point can, for example, be selected to be the midpoint in thepublic partial area or in one of these if there are several. The publicpartial area's reference point thus defines the origin in the localcoordinate system in which the locations of the private partial areasare expressed. Other unambiguously determined reference points can ofcourse also be selected, for example using features. The positions ofthe private partial areas are given as coordinates, for example for themidpoint in the private partial areas, in relation to the referencepoint. These coordinates are stored as part of the public part of thetemplate.

In step 34, the computer unit 22 then stores the template, awaitingtransmission to the smart card 24. The public part of the template willthus contain the public partial area(s) and coordinates for thelocations of the private partial areas in relation to a reference point.Its private part will contain the private partial areas. Comparisoncriteria can also be stored in the private part in the form of thresholdvalues for what degree of conformity is to be achieved in matching theprivate partial areas with partial areas of the current fingerprint inorder for the template and the current fingerprint to be considered tocome from the same individual. The partial areas are preferably storedin the form of bit maps.

Thereafter, in step 35, it is checked whether the recording of thereference fingerprint data has been completed. The criterion can, forexample, be that a predetermined number of alternative templates havebeen stored and/or that templates that correspond to a fingerprint areaof a predetermined size have been stored.

If the computer unit 22 finds that the recording has not been completed,a signal indicating this is given to the user, step 36. The signal can,for example, comprise an indication that the recording has not beencompleted, an indication of how large a part of the predetermined areahas been stored or an indication of how the user needs to move hisfinger in order to continue with the recording. Thereafter, the flowreturns to step 30 and the computer unit 22 awaits a new image beingrecorded.

If instead the computer unit 22 finds that the recording has beencompleted, all the recorded templates are transferred to and stored inthe memory on the smart card, step 37. They will then constitutealternative templates for one and the same finger and one and the sameperson for selective use during later identity checks.

Thereafter a signal is given to the user that the recording has beencompleted, step 38.

Reference fingerprint data has now been recorded for the user. As therecording was carried out under secure conditions and the user'sidentity is also recorded on the smart card, his identity is known.

In the embodiment described above, each recorded image is processedseparately and a template or a set of reference fingerprint data isdetermined for each image. The images thus do not need to overlap eachother and the coordinates for the private partial areas are expressed ina local coordinate system for each template where the public partialarea defines the origin. A variant of this is to construct thealternative templates successively. In such an embodiment, the firstimage can be recorded and processed in the way described above. When theprocessing of this image has been completed, there is thus stored afirst template that comprises a public partial area and a plurality ofprivate partial areas. When the second image has been recorded andaccepted in step 31, one or more of the partial areas in the firsttemplate are compared with the second image in order to check whetherany of the partial areas has a match in the second image so that therelative positions of the first and the second image can thereby bedetermined. If none of the partial areas in the first template has amatch in the second image, the user receives an indication that hisfinger is to be moved and a new image is to be recorded. If any one ofthe partial areas has a match, partial areas are extracted from thesecond image in the same way as for the first image. The position ofthese partial areas in relation to the reference point in the firstimage is now known, as the relative positions of the first and thesecond image have been established. The positions can thus be expressedin a coordinate system that is common to the first and the second imageand where, for example, a partial area extracted from the first imagedefines the origin. The partial areas from the second image are thusstored with their coordinates in the first template, after which a checkis carried out as to whether the recording has been completed. If suchis not the case, a third image is recorded and the method for the secondimage is repeated. When the recording is judged to have been completed,there is stored a single template with a large number of partial areas,of which at this stage only one is selected as a public partial area.Before storage on the smart cart is carried out in step 37, however, oneor more additional partial areas are selected as public partial areas.The partial areas can then be divided and stored on the smart card as anumber of separate templates, each of which contains a public partialarea and a plurality of private partial areas with associatedcoordinates. Each template thus contains a complete set of data that isto be used for verification. Certain data can, however, be identical fortwo templates, for example if two templates are allowed to have aprivate partial area that is the same for both templates. Alternatively,all the partial areas can be stored together as a “super template”,where different subsets form the alternative templates. Each such subsetconsists of a public partial area with a plurality of associated privatepartial areas. Certain private partial areas can thus belong to severalsubsets or templates, but are only stored once. In this case, thepartial areas can be stored in a table which states which partial areasare public and which are private and also gives the coordinates of therespective partial areas in the common coordinate system in which one ofthe public partial areas defines the origin.

In a further embodiment, the images are not processed sequentially, butfirst a number of recorded images or a selection of these are combinedinto one large image. The combining can be carried out in a known way,for example by calculation of a score on the basis of similarity betweenoverlapping pixels, or by means of some correlation technique. Partialareas are extracted from the combined image in the same way as describedfor the individual images above. One of the partial areas is selected asa public partial area with a reference point, in relation to which thepositions of other extracted partial areas can be determined. When allthe partial areas have been extracted, further partial areas can beselected as public partial areas. Both here and in the embodimentdescribed above, the additional partial areas are selected if possiblewith such density that regardless of in which position an image is takenof the fingertip, there will be one, but preferably not more than one,public partial area that matches in the image. The storage will then becarried out as separate templates or as one “super template”.

The advantage of storing the templates as subsets of a “super template”is that for a given memory size, a much larger number of alternativetemplates can be stored, as each partial area is only stored once.

In the following, it will be described how an identity check thatconsists of a verification can be carried out using the system in FIG.2, with reference to the flow chart in FIG. 4 and on the basis of thereference fingerprint data recorded by the method according to FIG. 3,which reference fingerprint data in this case thus includes partialareas that are stored on the smart card 24.

When a user wants to verify his identity, he places his smart card 24with the templates stored in the memory in the smart card reader 23 andplaces the same finger that was used when making the recording of thetemplate on the sensor 21.

When the user has placed his finger on the sensor 21, a current image isrecorded, step 40, of an area on the fingertip. The area can, forexample, be the area 3 in FIG. 1 c. The image constitutes a digitalrepresentation of this area.

In a following step 41, it is checked whether the current image can beaccepted. The check can comprise one or more of the elements mentionedabove in connection with step 31.

If the current image cannot be accepted, a signal to this effect isgiven to the user in step 42, in the same way as in step 32. Thereafterthe flow returns to step 40 and the computer unit 22 awaits therecording of a new image.

If, on the other hand, the current image can be accepted, the computerunit 22 reads the public parts of the templates stored on the smart card23, step 43. Each of the public parts comprises, as has been describedabove, a public partial area and coordinates for the location of theprivate partial areas.

In step 44, each of the public partial areas is compared or correlatedwith the recorded current image of the fingerprint area. The correlationcan be carried out against the whole image or against a part of this ofa predetermined size, preferably in this case in the center of theimage. During the correlation, the public partial area is “passed” overthe image and a pixel by pixel comparison is carried out in eachposition. If a pixel in the template partial area corresponds to a pixelin the image of the current fingerprint, then a particular value, forexample 1, is added to a total. If the pixels do not correspond, thenthe total is not increased. When the public partial area of the templatehas been passed over the whole image or the selected area of this, aposition is obtained where the public partial area of the templatecorrelates best with or overlaps the current fingerprint. The publicpartial area can also be rotated in relation to the image of the currentfingerprint in order to determine whether a better correlation can beobtained.

On the basis of the public partial area for which the highest pointtotal is obtained, in step 45 the template is selected or determinedthat is to be used for the final verification on the smart card. Thepoint total for the public partial area must, however, exceed apredetermined reference total, step 46. If this is not the case, theverification is considered to have failed and a signal to this effect isgiven to the user in step 47 in a corresponding way to that describedabove, after which the flow returns to step 40.

The correlation can fail for at least two reasons. Either the currentfingerprint does not come from the person or the finger from which thereference fingerprint has been recorded and then there is quite simplyno match with any of the public partial areas in the current image.Alternatively, the current fingerprint and the templates come from thesame person and the same finger, but the person in question is holdinghis finger in such a position in relation to the sensor 21 that none ofthe areas that correspond to the public partial areas lie within thearea of the sensor. As several alternative templates are stored, theprobability of this is, however, small.

If the templates are stored as separate templates and not as subsets ofa “super template”, it can, however, happen that the current area thathas been recorded does not fully correspond to any one of the templates,but, for example, overlaps two adjacent templates. Some of the privatepartial areas in the selected template will not then have any match inthe current image. The computer unit can determine that such is the caseusing the coordinates in the public part of the selected template, whichcoordinates thus indicate the location of the private partial areas inrelation to the public partial area. If such a case is detected, thecomputer unit can give a signal to the user to the effect that he is tomake a slight change in the position of his finger on the sensor, step48, which step is shown by dashed lines in order to indicate that thisstep is not always carried out. This signal then preferably comprises anindication of in which direction the slight change of position is tomade. When the image at the slightly changed position has been recordedby the sensor 21 and the computer unit 22 has checked that a sufficientnumber of the private partial areas have matches within the currentimage, partial areas of the current image are sent to the smart card 24for final verification, step 49. More specifically, a partial area of apredetermined size is selected in the current image around each pointthat is defined by the coordinates in the public part of the template.The partial areas in the current image can, however, be slightly largerthan corresponding partial areas in the template in order to compensatefor any deformation of the fingerprint if the finger is placed on thesensor with a different pressure when recording the current image.Together with the partial areas of the current image, an indication, forexample a number or a pair of coordinates, is also transmitted ifrequired, concerning which of the templates on the smart card 24 is tobe used for the final verification.

If the templates are stored as separate templates, the public part ofthe respective template contains the coordinates for all the privatepartial areas in the template.

If the templates are stored instead as subsets of a super template,coordinates can be associated with each public partial area for all theprivate partial areas in the super template or for a number of these. Itcan then happen that the coordinates for some of the private partialareas are such that corresponding areas of the finger of the checkedperson do not lie within the area of the sensor and accordingly are notfound in the current image which has been recorded. There is then nopartial area in the current image to send to the smart card. Random dataor noise can then be sent instead of the partial areas in order toindicate that the partial area is missing or alternatively only thepartial areas that are to be found within the current image may be sent,with an indication of which these areas are, for example by means of thecoordinates. In this case, it is thus determined which template, that iswhich of the subsets of the super template, is to be used for the datathat is returned to the smart card 24 from the computer unit 22. In thisconnection, it should be pointed out that the area of the current imagethat matches the public partial area can, but need not, be sent to thesmart card.

A final comparison is then carried out on the smart card 24 between theprivate partial areas in the selected template and the current partialareas in the current image that has been sent to the smart card. Thecomparison does not constitute a part of the present invention and istherefore not described here in more detail. Instead, reference is madeto the above-mentioned WO 01/84494.

Alternative Embodiments

Although a special embodiment has been described above, it will beobvious to those skilled in the art that many alternatives,modifications and variations are possible in the light of the abovedescription.

For example, the smart card 24 and the smart card reader 23 can be aseparate unit with which the computer unit 22 communicates. Thisseparate unit, which can be portable or stationary, does not need to bein the physical vicinity of the computer unit 22, but communication canbe carried out via, for example, a computer network, telecommunicationsnetwork or a cable. The separate unit can, for example, be anothercomputer, a mobile telephone or a PDA. The template can be stored in aprotected memory, that is a memory that is protected againstunauthorized access, in the separate unit.

In the description above, it has in addition been stated that when thecorrect template is to be determined during the identity check, thepublic partial areas of all the templates are compared against thecurrent partial fingerprint. This is not completely necessary. It ispossible to compare one public partial area at a time against thecurrent partial fingerprint and after each comparison to check whetherthe public partial area matches sufficiently well. If so, the comparisoncan be terminated and a template that contains that public partial areais used for the identity check. Another alternative is to includeinformation in the public part of each template concerning the relativepositions of the partial areas of other templates. When a public partialarea that matches the current partial fingerprint has been found, thetemplate with the public partial area that has the best location inrelation to the current partial fingerprint can be determinedimmediately. This is then the template that is to be used for the actualverification.

In the example above, three templates were stored. It is possible to usemore or fewer templates.

The method for checking identity can be used for both verification andidentification.

In the description above, it has been stated that the public part of thetemplate contains a partial area of a recorded fingerprint image.Alternatively, however, the public part of the template can contain aset of features, for example features that are obtained from the samearea of the finger as the private partial areas in the-template. Thesecan be used in a corresponding way to the public partial area. When itis to be determined which template is to be used for the identity check,the set of features in the public part of the template is thus sent tothe computer unit, in which the set is compared with the currentfingerprint image. The template for which the most matching features areobtained is then used for the identity check. The coordinates in thepublic part of the template, which for example can refer to a referencepoint in the form of a particular feature, are then used to determinewhich partial areas of the current fingerprint image are to be sent tothe smart card or some other unit where the identity check is to becarried out.

As an additional variant, features from the whole area of the fingerthat is covered by all the templates can be recorded and form publicdata that is common to all the templates. During the identity check,this public data is then sent from the smart card to the computer unitand is compared with features in the current image. Only certainfeatures in the public data will have matches in the current image, butby means of these the current image and the data on the smart card canbe registered with each other so that a suitable template can be usedfor the identity check.

FIG. 1 a shows that the areas overlap each other. This is not necessary,particularly if the user receives a signal with an indication concerninghow his finger is to be moved after a failed verification attempt.

In the method described in connection with the flow chart in FIG. 3, itis stated that the reference fingerprint data is recorded successivelyuntil a particular criterion is fulfilled. As an alternative, it couldbe possible to let the user move his finger around to a plurality ofdifferent positions on the fingerprint sensor, record an image for eachposition and, when a certain number of images has been recorded or aftera certain length of time, terminate the recording of the images, selecta predetermined number of images on the basis of the relativeoverlapping so that as good a spread as possible is obtained, anddetermine and store a set of reference fingerprint data for each of theselected images.

The methods according to the different embodiments and aspects of theinvention may be combined in different ways without departing from thescope of the appended claims.

1. A method for recording reference fingerprint data for a person,comprising the steps of: receiving (30) at least two fingerprint images(2 a, 2 b, 2 c) that depict at least partially different areas of oneand the same finger of the person, and storing (34) data from said atleast two fingerprint images (2 a, 2 b, 2 c) as at least two alternativesets of reference fingerprint data, that comprise data from at leastpartially different areas (A, B, C, D, E, F), of the finger, for use inchecking the identity of the person, whereby said at least twoalternative sets of reference fingerprint data, are stored in a firstunit (24) in which checking of the person's identity is to be carriedout, each of the sets of reference fingerprint data, comprising publicdata that can be communicated from the first unit (24) to a second unit(22) during the check, and also private data that is used exclusively inthe first unit (24) during the check.
 2. The method as claimed in claim1, in which the two sets of reference fingerprint data, are determinedfrom their respective fingerprint image of the at least two fingerprintimages (2 a, 2 b, 2 c).
 3. The method as claimed in claim 1, in whichthe step (37) of storing data as at least two alternative sets ofreference fingerprint data, comprises storing data from the first image,determining the relative positions of the first and the second images bycomparison of the data stored from the first image with the secondimage, and storing data from the second image utilizing the thusdetermined relative positions of the images.
 4. The method as claimed inclaim 1, in which the first and the second images are combined into alarger image and said at least two alternative sets of referencefingerprint data, are determined from the combined image.
 5. The methodas claimed in claim 1, in which the data that is stored from the imagescomprises a plurality of partial areas (A, B, C, D, E, F), of the images(2 a, 2 b, 2 c).
 6. The method as claimed in claim 1, in which the datathat is stored from the images comprises features.
 7. The method asclaimed in claim 1, in which the fingerprint images (2 a, 2 b, 2 c) arereceived from a sensor that has a sensor surface, the size of which onlyallows a partial fingerprint to be recorded.
 8. The method as claimed inclaim 1, further comprising the step of checking whether said at leasttwo fingerprint images (2 a, 2 b, 2 c) together cover an area of thefinger that is larger than a predetermined area and, if so, giving (38)an indication to the person that the recording has been completed. 9.The method as claimed in claim 7, further comprising the step of giving(32) an indication to the person concerning how the fingertip is to bemoved in relation to the sensor.
 10. The method as claimed in claim 1,comprising the steps of receiving more than two fingerprint images (2 a,2 b, 2 c), checking how these images overlap each other and selecting apredetermined number of images on the basis of the overlap, the storageof said at least two alternative sets of reference fingerprint databeing carried out from the selected images.
 11. A computer programcomprising program code that carries out a method as claimed in claim 1when it is executed in a computer.
 12. A computer-readable computerprogram product on which is stored a computer program as claimed inclaim
 11. 13. A device for recording reference fingerprint data for aperson, comprising a signal processor (22) which is arranged to receive(30) at least two fingerprint images (2 a, 2 b, 2 c) depicting at leastpartially different areas of one and the same finger of the person, andto store data from said at least two fingerprint images as at least twoalternative sets of reference fingerprint data, comprising data from atleast partially different areas (A, B, C, D, E, F), of the finger, foruse in checking the identity of the person, whereby said at least twoalternative sets of reference fingerprint data, are stored in a firstunit (24) in which checking of the person's identity is to be carriedout, each of the sets of reference fingerprint data, comprising publicdata that can be communicated from the first unit (24) to a second unit(22) during the check, and also private data that is used exclusively inthe first unit (24) during the check.
 14. The device as claimed in claim13, which device comprises a sensor (21) that has a sensor surface, thesize of which only allows a partial fingerprint to be recorded.
 15. Amethod for checking identity, comprising the steps of: receiving (40) acurrent fingerprint image (3) that depicts a part of a finger of aperson whose identity is to be checked; comparing (44) data from thefingerprint image (3) with data from at least a first set of referencefingerprint data selected from at least two previously storedalternative sets of reference fingerprint data that relate to at leastpartially different areas of one and the same finger from a person whoseidentity is known; and, on the basis of the result of the comparison,determining (46) which of said at least two alternative sets ofreference fingerprint data is to be used for the identity check, wherebysaid at least two alternative sets of reference fingerprint data arestored in a first unit (24) and comprise public data that can becommunicated from the first unit (24) to a second unit (22) during theidentity check, and private data that is used exclusively in the firstunit (24) during the check, and in which the step of comparing (44)comprises receiving public data from the first unit and comparing thiswith the data from the fingerprint image (3).
 16. The method as claimedin claim 15, in which the data from the fingerprint image (3) iscompared with data from all the sets of reference fingerprint data inorder to determine which set of reference fingerprint data is to be usedfor the identity check.
 17. The method as claimed in claim 15, in whichthe first set of reference fingerprint data comprises at least onepartial area (A, B, C, D, E, F) of a previously recorded fingerprintimage (2 a, 2 b, 2 c) of a part of a finger of the person whose identityis known, and in which the step of comparing (40) comprises checkingwhether the partial area has a match in the current fingerprint image(3).
 18. The method as claimed in claim 15, in which the first set ofreference fingerprint data comprises a plurality of reference features,and in which the step of comparing (44) comprises comparing referencefeatures with features in the current fingerprint image (3).
 19. Themethod as claimed in claim 15, further comprising the step (42) ofgiving an indication to the person whose identity is to be checkedconcerning how his finger is to be moved in relation to the sensor (21).20. A computer program comprising program code that carries out a methodas claimed in claim 15 when it is executed in a computer.
 21. Acomputer-readable computer program product on which is stored a computerprogram as claimed in claim
 20. 22. A device for checking identity,comprising a signal processor which is arranged to: receive (40) acurrent fingerprint image (3) that depicts a part of a finger of aperson whose identity is to be checked; compare (44) data from thefingerprint image (3) with data from at least a first set of referencefingerprint data selected from at least two previously stored sets ofreference fingerprint data that relate to at least partially differentareas of a finger of a person whose identity is known; and determine(45) which of said at least two sets of reference fingerprint data is tobe used for the identity check, whereby said at least two previouslystored sets of reference fingerprint data are received from a first unit(24) and comprise public data that can be communicated from the firstunit (24) to a second unit (22) during the identity check, and privatedata that is used exclusively in the first unit (24) during the check,and in which the step of comparing (44) comprises receiving public datafrom the first unit and comparing this with the data from thefingerprint image (3).
 23. The device as claimed in claim 23, whichdevice comprises a sensor (21) that has a sensor surface, the size ofwhich only allows a partial fingerprint to be recorded.
 24. A method forrecording reference fingerprint data for a person, comprising the stepsof receiving (30) at least two fingerprint images (2 a, 2 b, 2 c) thatdepict at least partially different areas of one and the same finger ofthe person, and storing (37) data from said at least two fingerprintimages as at least two alternative sets of reference fingerprint data,that comprise data from at least partially different areas (A, B, C, D,E, F), of the finger, for use in checking the identity of the person.25. A method for checking identity, comprising the steps of receiving(40) a current fingerprint image (3) that depicts a part of a finger ofa person whose identity is to be checked; comparing (44) data from thefingerprint image with data from at least a first set of referencefingerprint data selected from at least two previously storedalternative sets of reference fingerprint data that relate to at leastpartially different areas of one and the same finger from a person whoseidentity is known; and, on the basis of the result of the comparison,determining (45) which of said at least two alternative sets ofreference fingerprint data is to be used for the identity check.
 26. Themethod as claimed in claim 8, further comprising the step of giving (32)an indication to the person concerning how the fingertip is to be movedin relation to the sensor.
 27. The method as claimed in claim 16, inwhich the first set of reference fingerprint data comprises at least onepartial area (A, B, C, D, E, F) of a previously recorded fingerprintimage (2 a, 2 b, 2 c) of a part of a finger of the person whose identityis known, and in which the step of comparing (40) comprises checkingwhether the partial area has a match in the current fingerprint image(3).
 28. The method as claimed in claim 16, in which the first set ofreference fingerprint data comprises a plurality of reference features,and in which the step of comparing (44) comprises comparing referencefeatures with features in the current fingerprint image (3).
 29. Themethod as claimed in claim 17, in which the first set of referencefingerprint data comprises a plurality of reference features, and inwhich the step of comparing (44) comprises comparing reference featureswith features in the current fingerprint image (3).
 30. The method asclaimed in claim 16, further comprising the step (42) of giving anindication to the person whose identity is to be checked concerning howhis finger is to be moved in relation to the sensor (21).
 31. The methodas claimed in claim 17, further comprising the step (42) of giving anindication to the person whose identity is to be checked concerning howhis finger is to be moved in relation to the sensor (21).
 32. The methodas claimed in claim 18, further comprising the step (42) of giving anindication to the person whose identity is to be checked concerning howhis finger is to be moved in relation to the sensor (21).
 33. A computerprogram comprising program code that carries out a method as claimed inclaim 16 when it is executed in a computer.
 34. A computer programcomprising program code that carries out a method as claimed in claim 17when it is executed in a computer.
 35. A computer program comprisingprogram code that carries out a method as claimed in claim 18 when it isexecuted in a computer.
 36. A computer program comprising program codethat carries out a method as claimed in claim 19 when it is executed ina computer.